Author Topic: Falcon 1 Block 5?  (Read 6021 times)

Offline billh

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Falcon 1 Block 5?
« on: 11/22/2017 04:08 pm »
Just for fun: what would the payload of an uprated Falcon 1 be if you used a Block 5 Merlin on the first stage? How much would you stretch the stages? This is not a serious proposal. I'm just musing about how far SpaceX has come in their engine development.

Offline nacnud

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Re: Falcon 1 Block 5?
« Reply #1 on: 11/22/2017 04:13 pm »
Is changing the diameter of the first stage within the rules? And what 2nd stage engine(s) can be used?

Offline billh

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Re: Falcon 1 Block 5?
« Reply #2 on: 11/22/2017 04:18 pm »
Is changing the diameter of the first stage within the rules? And what 2nd stage engine(s) can be used?
I was thinking of sticking with the Kestrel and the current diameter. But since we're just having fun there's no reason not to consider other options.

Offline GreenShrike

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Re: Falcon 1 Block 5?
« Reply #3 on: 11/22/2017 11:16 pm »
Merlin 1D+ is 845kN, which is ~84 tonnes of thrust. So with a liftoff thrust/weight ratio of 1.15, the rocket can mass 73t at liftoff. Jumping ahead, assume the payload is 1.25t, so the rocket itself is 71.75t.

Because the Kestrel is small and pressure fed and generally non-optimal, we'll make S1 bigger to compensate -- dedicate 90% of the rocket's mass to S1. This is about what Falcon 1e would have done, from its Wikipedia page.

So, S1 is 64.61t. If SpaceX does a good job with PMF and gets it to 0.94 (slightly better than F1e), that's 3.88t dry with 60.74t propellant.

With a vacuum ISP of 311s and sea-level ISP of 282s, and using the rule of thumb that you can use the SL ISP + 2/3rds of the difference between vac and SL as an average ISP for S1's flight (or something like that, anyway), that average ISP is 301.33s.

So that's enough information to calculate S1's Delta-V: (301.33*9.8) * ln(73/(73-60.74)) = ~5270m/s.  Hey -- over halfway to orbit.


10% of the rocket is S2, or 7.18t. A pressure fed design requires thick walls to handle the tank pressure, so S2's PMF will not be good: call it 0.86 (F1e's was to be ~0.88). That means a dry mass of 1.01t and 6.17t of prop.

I'm going to go all wild and crazy and say that SpaceX improved the Kestrel like they were planning, and squeezed out an ISP improvement, from 317s to 325s.

S2's Delta-V is then (325*9.8) * ln ((7.18 + 1.25)/((7.18 + 1.25) - 6.17)) = ~4190m/s.

S1 dV + S2 dV = ~9460m/s

~9.5km/s should put 1.25t somewhere in LEO. Don't ask me where.

1.5t payload reduces performance to ~9.2km/s, and 1.75t to ~8.9km/s. Those may or may not reach somewhere in LEO.


On a side note, replacing the Kestrel with a couple of electro-pumped Rutherfords (Vac ISP 333) to eliminate the heavy pressurized tanks and increase PMF to .93, and adjusting S2 to 15% of the rocket's mass, changes performance to ~9.4km/s with 2t of payload. Poor PMF is a performance killer in an orbital stage.


I just threw together the attached spreadsheet, so there's probably errors. And almost certainly bad assumptions. Take it for what it's worth and corrections welcome. ;-)

Edit: For some crazy reason, people are actually downloading the spreadsheet, so I've updated it with more explanations and such. Hopefully, you now need be less of a mind reader to actually play around with it.
« Last Edit: 11/23/2017 04:32 pm by GreenShrike »
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Offline Comga

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Re: Falcon 1 Block 5?
« Reply #4 on: 11/27/2017 04:41 am »
This appears to be a revival of an old thread on hypothetical updates to the Falcon 1.  All that's new is the "Block 5" Merlin 1D. 

One reason (among many) the Falcon 1 was abandoned was that Musk's initial idea for recovery, parachuting into the ocean, didn't work.  More interesting to me than the payload increase for a M1D F1 would be a recoverable F1.  How could recovery elements from the Falcon 9, be incorporated to recover much of, and reduce the launch cost of, a single Merlin engine rocket?

The first problem is throttling down to land the first stage.  The 9:1 advantage of the Falcon 9 clustering would have to be replaced with some other lower thrust LOX-RP-1 engines.  How about two Rutherfords, or XR-5k18 engines from the remains of XCOR?

And the second stage?   Again the 9:1 advantage is lost.  Using the same main engine as the first stage doesn't make sense. Would one or more of the small engines work?

So if we add the first stage recovery subsystems (landing engines, legs, grid fins, ACS thrusters), with the concomitant payload reduction, and the smaller second stage, what kind of performance would be left for a  mostly recoverable M1D powered rocket?
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Offline Giovanni DS

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Re: Falcon 1 Block 5?
« Reply #5 on: 11/27/2017 05:12 pm »
I think, more than recovery, this hypothetical new F1 could be interesting to make a final launch with engines that reached the end of their operating life.

That would keep cost down, the engine is basically already paid for.

Giovanni

Offline intrepidpursuit

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Re: Falcon 1 Block 5?
« Reply #6 on: 11/27/2017 09:46 pm »
It was never clear to me at what point the parachute recovery option failed. ISTM that the reentry burn could be done on an F1 with a low throttle setting, requiring replacing the landing burn with a parachute. A bouncy castle seems out of the question, so it would have to survive a dunk in the salt water. Perhaps an inflatable at the base combined with pressurized tanks so it lands on its engine and tilts to float on its side with with engine out of the water?

Those seem like surmountable problems to me. S2 really has to stay a Kestrel or it isn't an F1 anymore. S2 won't be recoverable, but will be cheap.

Iridium is probably a good measure. Could their sats go up on dedicated launches for a similar price? Could F1 compete with Electron? If not then it probably isn't a viable alternative.

Offline Comga

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Re: Falcon 1 Block 5?
« Reply #7 on: 11/27/2017 10:37 pm »
It was never clear to me at what point the parachute recovery option failed. ISTM that the reentry burn could be done on an F1 with a low throttle setting, requiring replacing the landing burn with a parachute. A bouncy castle seems out of the question, so it would have to survive a dunk in the salt water. Perhaps an inflatable at the base combined with pressurized tanks so it lands on its engine and tilts to float on its side with with engine out of the water?

Those seem like surmountable problems to me. S2 really has to stay a Kestrel or it isn't an F1 anymore. S2 won't be recoverable, but will be cheap.

Iridium is probably a good measure. Could their sats go up on dedicated launches for a similar price? Could F1 compete with Electron? If not then it probably isn't a viable alternative.

If you wish, but...

Being religious about the F1 with Kestrel seems besides the point.  So is sticking with the original diameter, which wasn't hard limited like the Falcon 9.

IMO the main point is to build a recoverable single engine Falcon.  An expendable rocket probably won't compete.
(Please respect the previous discussion of the existence or non-existence of "end of life" Merlin engines and don't rehash it.)

The parachute recovery failed when the stages broke up during uncontrolled hypersonic tumbling.  This could be solved by cold gas thrusters. 

The next challenge would be peak heating and stress.  F9 solves this with the reentry burn.  Not sure how this would work for an F1 with one third the thrust (1 engine instead of 3) but probably a smaller fraction of the mass.

Then there are the gridfins to control the atmospheric flight.

And lastly there is the "hoverslam" maneuver and the legs.

If all of these were scaled to preserve first stage recovery, what would be the payload capacity?
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Offline Giovanni DS

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Re: Falcon 1 Block 5?
« Reply #8 on: 11/28/2017 07:46 am »
IMO the main point is to build a recoverable single engine Falcon.  An expendable rocket probably won't compete.
(Please respect the previous discussion of the existence or non-existence of "end of life" Merlin engines and don't rehash it.)

Sorry but nowhere in the topic and opening post there is a reference to a recoverable F1.

About EOL, at some point they will have to decide to withdraw engines from the reuse loop, so non-existence can be true now but not necessarily true in few years.

Probably the F1 is not a priority for SpaceX but I imagine that a spin off company could manage the small launcher and keep it out of the main SpaceX activities. It would succeed or fail without affecting the parent.
« Last Edit: 11/28/2017 07:47 am by Giovanni DS »

Offline speedevil

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Re: Falcon 1 Block 5?
« Reply #9 on: 11/28/2017 11:43 am »
About EOL, at some point they will have to decide to withdraw engines from the reuse loop, so non-existence can be true now but not necessarily true in few years.

Probably the F1 is not a priority for SpaceX but I imagine that a spin off company could manage the small launcher and keep it out of the main SpaceX activities. It would succeed or fail without affecting the parent.

It's also plausible EOL for engines may happen not when they hit a cycle limit, but when BFR starts to work well enough to convince everyone to move over.
Aggressive testing could lead to BFR/S individual vehicles hitting broadly comparable flight numbers to the whole F9 system very fast.

Depending on your assumptions about cost, and availability, there may be a place for a F1-like launcher to compliment.
There is also the option then for recovery in that you stick an ion engine on it, and have it come to a common orbit for later pickup.

In principle there is also the silly idea of using it as a third and fourth stage for BFR.
Elon did say it fit.

Offline Giovanni DS

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Re: Falcon 1 Block 5?
« Reply #10 on: 11/28/2017 12:38 pm »
It's also plausible EOL for engines may happen not when they hit a cycle limit, but when BFR starts to work well enough to convince everyone to move over.

It is possible, personally I doubt that something so huge like BFR/S can be a single solution for all problems, I think there will still be use cases where the F9 would fit better even if not fully recoverable.

Launch a huge number of satellites to build a constellation is a thing, launching commercial sats is something different, F9 is here to stay for a long time IMHO.

Offline speedevil

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Re: Falcon 1 Block 5?
« Reply #11 on: 11/28/2017 01:19 pm »

It is possible, personally I doubt that something so huge like BFR/S can be a single solution for all problems, I think there will still be use cases where the F9 would fit better even if not fully recoverable.

Launch a huge number of satellites to build a constellation is a thing, launching commercial sats is something different, F9 is here to stay for a long time IMHO.

That is another way of saying 'I don't believe BFR can be reused over 35 times' or so. (assuming $600M cost).

If it can, then the cost of launching even smallsats crashes below the cost of F9S2.

F9 and a hypothetical F1B5 at that point would be of use for a 'assured access to space' point of view for small payloads, in the event of a BFR/S fleet grounding.

There is then only a small range of BFR performance between 'gets cancelled' (say a failure more than once every 4 flights), and 'competitive in the market for smallsat launches' at perhaps every 20 launches that makes F9 viable with BFR flying.

F1B5 remains competitive for a while longer.
« Last Edit: 11/28/2017 01:21 pm by speedevil »

Offline rakaydos

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Re: Falcon 1 Block 5?
« Reply #12 on: 11/29/2017 04:10 am »
Personally, I'd love to see a Raptor 1.

A single sea level raptor pushing an upper stage that uses the pressure-fed methalox RCS thruster as main propulsion.

The lower stage hoverslams into a crashnet, and the upper stage is heat shielded and small enough to parachute easilly.

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